Elasticache reserved node size flexibility not appliying to different sizes.

I apologize for the confusion, but there seems to be a misunderstanding about how the size flexibility for Amazon ElastiCache reserved nodes works.

The size flexibility feature does allow you to apply reserved node discounts across different sizes within the same node family, but there's a specific order in which the discounts are applied.

According to the ElastiCache reserved node size flexibility rules, the reserved node price will automatically apply to any running smaller node types within the node family before being applied to larger node types. In your case, this means that your six cache.m6g.large reserved nodes will first be applied to the five cache.m6g.large instances you're running.

After covering the five cache.m6g.large instances, you have one cache.m6g.large reserved node left. This remaining reserved node can indeed be applied to a larger instance, but it will only cover a portion of a larger instance's usage.

A cache.m6g.xlarge instance is twice the size of a cache.m6g.large instance. So, your remaining cache.m6g.large reserved node would only cover 50% of one cache.m6g.xlarge instance's usage.

In your specific scenario, you're seeing savings on your five cache.m6g.large instances, which is correct. The sixth reserved node is likely being applied to one of your cache.m6g.xlarge instances, but it's only covering half of its usage. This partial coverage might not be immediately apparent in your billing, which could explain why you're not seeing the expected savings on the larger instances.

If you want full coverage for your two cache.m6g.xlarge instances, you would need to purchase additional reserved nodes equivalent to their size. Remember, the system automatically applies the discounts to smaller instances first, then to larger ones if there's remaining capacity.

If you're still seeing unexpected behavior after considering this explanation, I would recommend contacting AWS Support for a detailed analysis of your specific account and usage patterns.
Sources
New – Size flexibility for Amazon ElastiCache reserved nodes | AWS Database Blog
AWS announces Reserved Nodes flexibility for Amazon ElastiCache - AWS